A roundness measuring machine is often used to measure the form deviation of a measured sphere from a perfect sphere (i.e., sphericity). In the industry standard of a “Rolling bearings-Balls” in Japanese Industrial Standards, the sphericity is defined as “The greatest radial distance, in any equatorial plane, between the smallest circumscribed sphere and the greatest inscribed sphere, with their centres common to the least square sphere centre”. And information about the measurement of the sphericity of the steel ball is given in annex A of above standard. It is prescribed that “the measurement of the sphericity is carried out by the roundness measuring on three equatorial planes at 90 degrees with each other. The roundness measuring on one equatorial plane to be measured is carried out with its centre common to the least square sphere centre. The roundness in the equatorial plane to be measured is the maximum value of difference in radius in each equatorial plane to be measured” (JIS B1501(2009), Chapter 3.6.1 and Annex A corresponding to IS03290(2001), Annex A). Also in Chapter 3.2 of Specific Application Documents for Technical Requirements (Form Measuring Machine/Sphere) shown in JCT20116 issued by JCSS, the roundness of a sphere is defined as “the maximum value of roundness measured along the line of intersection with the surface of the sphere and each of three planes which pass through the center of sphere and are substantially orthogonal to each other”. Any of the definitions require the measurement of contours, with the roundness measuring machine, along the line of intersection with the surface of a sphere and each of three planes substantially orthogonal to each other.
To achieve this evaluation method, it is necessary to perform an evaluation composed of the following three steps: firstly, aligning a ball in the center of turntable of a roundness measuring machine and then measuring a contour along one of the lines of intersection; secondly, changing the posture of the held ball so that the line of intersection to be measured next is substantially orthogonal to the previously measured line of intersection, and measuring the contour after alignment; and finally, repeating the same operation with the second step.
If such a series of evaluation procedure is conducted manually, the operation will become complicated and difficult because the motion of changing the posture of the held ball twice needs to be performed manually and accurately. Even when a posture controlling device which automatically changes the posture of the held ball is used together with a roundness measuring machine, the development of such mechanism is difficult because it is necessary to devise the handling of ball and have a design in which any physical interference between the posture controlling device and the roundness measuring machine should be avoided (see Japanese Unexamined Patent Publication No. 2000-292138).